Strain 121 (Geogemma barossii) is a single-celledmicrobe, of the domainArchaea. First discovered 200 miles (320 km) off Puget Sound in a hydrothermal vent, it is a hyperthermophile, able to survive and reproduce at 121 °C (250 °F), hence its name. It was (at the time of discovery) the only known form of life that can tolerate such high temperatures. 130 °C (266 °F) is proven to be only biostatic for Strain 121, meaning that although growth is halted, the archaeum remains viable, and can resume reproducing once it has been transferred to a cooler medium.

The ability to grow at 121 °C is significant because medical equipment is exposed to this temperature for sterilization in an autoclave. Prior to the 2003 discovery of Strain 121, a fifteen-minute exposure to autoclave temperatures was believed to kill all living organisms. However, Strain 121 is non-infectious because it cannot grow at temperatures near 37 °C.